This invention relates to a method for producing powder of .alpha.-silicon nitride and more particularly to a method of producing powder of .alpha.-silicon nitride of high quality in high yield.
A sintered product of powder mixture containing powder of silicon nitride as a main raw material such as silicon nitride-yttrium oxide (Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3) or silicon nitride-magnesium oxide (Si.sub.3 N.sub.4 --MgO) has high mechanical strength and heat resistance. Therefore, an attempt is made to apply such sintered product as a material of a structural member of, for example, a high temperature gas turbine. A sintered product practically used as a material of a structural member withstanding high temperature and great stress is rigidly demanded to have prominent physical and chemical stability at high temperature. The particularly important thermal and mechanical properties of the sintered product are largely affected by the kinds of the powders and the content of impurities therein. The powder of silicon nitride is preferred to contain as much .alpha.-Si.sub.3 N.sub.4 as possible. The following three proceses are generally known for the production of powder of Si.sub.3 N.sub.4 :
(1) Nitrogenizing powder of metallic silicon; EQU 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4
(2) Carrying out a gaseous reaction between silicon tetrachloride or silane and ammonia; EQU 3SiCl.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12HCl
(3) Reducing silica powder with carbon in a nitrogen atmosphere EQU 3SiO.sub.2 +6C+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6CO
With the above-mentioned process of (1) in which an exothermal reaction takes place, it is necessary to control the exothermal reaction, for example, by finely crushing relatively coarse powder of silicon after nitrogenization. During the step of such fine crushing, impurities are unavoidably carried into powder Si.sub.3 N.sub.4 produced. Therefore, the product has many problems for application as a material of, for example, a structural member for a high temperature gas turbine, though usable as a heat-resisting and corrosion-resisting material of low quality.
The process of (2) is suitable for production of a material used as a coating on the surface, of for example, a semiconductor element, but meets with a difficulty for the mass production of an inorganic heat-resistant material.
The process of (3) has the drawbacks that it is necessary to use fully purified powders of SiO.sub.2 and carbon as raw materials, and that since the resultant product is a mixture, of for example, .alpha.-Si.sub.3 N.sub.4, .beta.-Si.sub.3 N.sub.4, silicon oxynitride (Si.sub.2 ON.sub.2) and SiC, it is difficult to control a yield of .alpha.-Si.sub.3 N.sub.4. Therefore, the process of (3) has practically problems in raw material and reaction conditions, though having the advantage of causing reaction to proceed by an uncomplicated process.